Substrates such as glass are used in a multitude of applications ranging from commercial buildings, homes, automobiles, appliances, etc. The substrates are often coated with functional coatings to obtain the desired performance attributes.
A wide variety of functional coatings are known in the art including, but not limited to, electroconductive coatings, solar control coatings, photocatalytic coatings, low emissivity coatings, transparent conductive coatings, etc. An example of a functional coating is a metallic based high transmittance, low emissivity coating that includes at least one metallic layer(s) sandwiched between layers of dielectric material. Usually, the metallic layer is gold, copper, or silver, and the dielectric material is a metal oxide such as tin oxide, indium oxide, titanium oxide, bismuth oxide, zinc oxide, zirconium oxide or zinc/tin oxide.
For certain applications, it is necessary to heat a substrate coated with a functional coating. For example, a coated glass substrate that will be used as an automotive windshield may need to be heated to bend the glass. Typically, glass will be heated for 20–30 minutes to a maximum temperature of 1150° F. to 1200° F. to accomplish the necessary bending for an automotive windshield. Depending on the complexity of the bend, the temperatures could be higher and the duration longer.
Heating a coated substrate can be problematic if the coating contains a layer(s) that will degrade upon heating. Generally, heating a coated substrate will produce beneficial results up to a certain temperature (for a certain duration of time) for various reasons, for example, mobile species becoming mobile upon heating and flowing out of certain coating layers, but then adverse affects arise. The combination of temperature and exposure time to which a coating layer can be heated before the performance of the coating starts to degrade is referred to herein as the “heat budget” of the coating. The performance of a coating starts to degrade after its heat budget is exceeded because at least one layer of coating will start to degrade. Every coating layer in a coating stack has a different heat budget that depends on the materials used to make the coating. The heat budget for a coating stack is determined by the layer of coating in the stack with the lowest heat budget at which the layer starts to degrade.
For example, in the high transmission, low emissivity coating as described above, the metallic layer(s) typically has the lowest heat budget in the coating stack. When a glass substrate coated with such a coating is exposed to heating conditions typically associated with bending, e.g. 1150° F. to 1200° F. for a period of 20 to 30 minutes, the metallic layer(s) will degrade. The degradation of the metallic layer(s) can result in a coated substrate with reduced optical and/or solar control properties. Specifically, the functional coating can demonstrate increased electrical resistivity, increased haze, decreased solar infrared (IR) reflectance, decreased visible light transmittance, increased emissivity, etc.
In addition to heating, other things can cause degradation of layers in a functional coating, such as exposure to certain chemicals including, but not limited to halides such as salt, chlorides, sulfur, chlorine, alkali, and enamels.
To ensure optimal performance of a coated substrate, it is desirable to protect any degradable coating layer(s) in a coating stack from conditions and/or substances that would result in degradation of the coating layer and subsequent decreased performance of the coated substrate. Conventionally, sacrificial layers like primer layers (also known as “blocker layers”) have been added to coating stacks, such as metallic based high transmission, low emissivity coatings, or applied at thicker levels to protect a degradable layer(s). The sacrificial layers preferentially respond to or react with the undesirable condition so as to protect other selected layers in the coating stack. The problem with adding a primer layer(s) or using a thicker layer(s) of primer is that after the coating is heated, excess primer can lead to poor adhesion due to failure at the interfaces of individual layers of coating and increased haze. Also, excess primer can make the coating soft and susceptible to damage by rubbing.
The present invention provides a coating composition having at least one layer of barrier coating to protect any degradable layer(s) in the coating stack. Coating compositions according to the present invention exhibit an increased heat budget and improved ability to withstand chemical corrosion.